Traditionally, fuel cells operate on the premise that a material having activity such as hydrogen, which is found in, for example, LNG, LPG or methanol, is oxidized through an electrochemical reaction, releasing chemical energy, which is converted into electrical energy. Compared to conventional methods of generating electricity, fuel cells are efficient and emit almost no air pollutants. Therefore, fuel cells are considered a technology of the future. As a result, research is being performed on ways to apply fuel cells as an alternative power source for vehicles so that energy may be conserved and air pollution minimized, resulting in less of an impact on global warming.
However, in the case where only fuel cells are used as the power source for a vehicle, various problems result. First, although fuel cells are able to maintain optimal efficiency in a particular range of power density (W/cm2), overall efficiency is lacking since the vehicle frequently falls out of this range of operation. Further, an electric motor that drives the fuel cell vehicle requires a higher direct current voltage in a high-speed range than when driving at low speeds. Since a characteristic of the fuel cells is an abrupt reduction in output voltage with an increase in output current, the fuel cells are unable to supply the high DC voltage required by the electric motor at high speeds. As a result, the high-speed performance of the motor deteriorates. In addition, fuel cells utilize a chemical reaction between hydrogen and oxygen to generate electrical energy. As a result, sufficient power is unable to be instantaneously supplied if there is an abrupt demand for power by the vehicle. Also, fuel cells have only the characteristic of outputting power, such that regenerative power, which is generated during braking of the drive motor of the vehicle, is unable to be absorbed. Hence, the efficient use of energy is limited.
To overcome the limitations of the output characteristics of fuel cells, a secondary energy source is used in combination with the fuel cells in an effort to compensate for these deficiencies. That is, a hybrid configuration is used.
The power system for a conventional fuel cell hybrid electric vehicle includes a fuel cell used as the main power source and a battery used as the auxiliary power source. A bi-directional DC/DC converter connected in parallel between the fuel cell and the battery, which supplies a stable voltage to a motor. This maintains a balance between different output voltages of the fuel cell and the battery and supplies a surplus voltage of the fuel cell and the regeneration energy as a charge voltage of the battery. An inverter connected to an output terminal of the bi-directional DC/DC converter and an output terminal of the fuel cell. The inverter also controls the operation of the motor by IGBT switching effected through PWM (pulse width modulation) control.
The DC/DC converter performs a buck operation, in which internal transistors undergo switching according to control signals applied from a processor such that power is transmitted from the high voltage source of the battery to the low voltage source of the fuel cell by the flow of current through diodes. The DC/DC converter also performs a boost operation, in which power is transmitted from the low voltage source of the fuel cell to the high voltage source of the battery.
In such a fuel cell hybrid electric vehicle that uses both a main energy source (fuel cell) and an auxiliary energy source (battery), a systematic and efficient method of distributing energy is needed between the two energy sources. However, in the conventional fuel cell hybrid electric vehicle, suitable power distribution between the two energy sources does not occur. There is an inefficient use of electric power, and energy required to drive the vehicle is unable to be supplied in a stable manner. Further, there are problems in recovering the regeneration energy, which is promoted as one of the main advantages of electric vehicles.